1. Field of the Invention
The present invention pertains to the art of sensors and, more particularly, to magnetic field sensors, such as Hall effect sensors, employed in measuring a rotational velocity of an element in environments where contamination is present, particularly ferrous contamination such as in automatic transmissions.
2. Discussion of the Prior Art
Magnetic field sensors are widely used to sense relative motion between objects, generally by detecting a change in magnetic flux. For instance, when it is desired to sense the rotational speed or position of a rotational shaft, a trigger wheel or disk is often mounted to the shaft and the wheel is provided with surface features for causing variations in magnetic flux. Typically, a magnet is provided to generate the magnetic flux and a sensor is placed opposite the trigger wheel. As the trigger wheel moves, the sensor will produce a signal with a strength that varies based on the strength of the sensed magnetic field. The signal inherently has a waveform that correlates to the shape and spacing of the surface features and the speed of rotation of the wheel and attached shaft. As such, the rotational speed and position of the shaft can be determined from the signal's waveform.
A typical Hall effect sensor is described in U.S. Pat. No. 7,816,772. As shown in FIG. 1 of the '772 patent, a Hall effect sensor includes a Hall IC having first and second Hall elements disposed on a leadframe. A concentrator is disposed on the other side of leadframe opposite the Hall elements. A magnet is connected to the concentrator and the whole assembly is over-molded with plastic. The sensor is placed in proximity to a gear with teeth that change the magnetic flux caused by the magnet as the gear rotates. While such sensors are perfectly adequate in most circumstances, problems develop when they are used in environments that contain ferrous contamination such as that found within an automatic transmission.
Automatic transmissions provide multiple speed ratios between an input from an engine and an output to driving wheels of a vehicle. Usually, an automatic transmission contains a torque converter arrangement with an impeller, a stator and a turbine. Power generated in an engine is directed to the impeller, transferred to the turbine and then exits the turbine through a turbine output shaft. Therefore, the turbine output shaft leads to a series of planetary gearsets which provide multiple speed ratios by using clutches and brakes to connect or brake different parts of the planetary gearsets to establish each desired rotational speed ratio. Power exits the transmission via a transmission output shaft and ultimately proceeds to the driving wheels. Hall effect rotational speed sensors are used in transmissions to measure the rotational speeds of parts of the transmission for several reasons, for example, controlling shifts from one rotational speed ratio to another. Measuring the speed of the turbine shaft and the transmission output shaft are particularly useful in controlling shift changes. A signal from the Hall effect rotational speed sensor is sent to a transmission control system which directs the speed and timing of engagement of the appropriate clutches or brakes to change rotational speed ratios while minimizing shift shock.
Automatic transmissions generate heat and need lubrication. To address these requirements, the transmission components, such as the torque converter and planetary gearsets, are mounted in a housing and a combination of lubricating and cooling oil is circulated around the various transmission components. Through natural wearing, small ferrous objects, such as metal pieces or filings, collect in the transmission casing. Hall effect rotational speed sensors mounted in the transmission casing and positioned to measure rotational shaft speed are sometimes affected by the ferrous contamination. Specifically, small particles of ferrous contamination are attracted by the magnet in the Hall effect sensor and position themselves on the face of the sensor. For example, again utilizing the arrangement disclosed in U.S. Pat. No. 7,816,772 as an example, ferrous contamination can develop between the gear teeth and the Hall effect sensor resulting in the Hall elements producing false readings, in addition to the signals produced as each tooth passes the sensor. The ferrous particles actually move across the face of the sensor as the particles follow the teeth. The false readings trigger a fault in the control system for the transmission and result in the vehicle being brought in for repair at a vehicle dealership.
A graph 1 showing output signals including a square wave signal 2 and an analog signal 4 from such a sensor is shown in FIG. 6. Note particularly that, instead of producing a regular pattern at a constant sensed velocity, square wave signal 2 exhibits dropouts 6 and duty cycle irregularities 8, while analog signal 4 shows irregularities 9. A mechanic usually wipes the sensor clean and/or replaces the sensor, thus temporally stopping the fault. Customers and repair facilities often spend much time and expense replacing sensors only to find the next sensor is affected by another collection of ferrous particles.
Based on the above, there exists a need in the art for a rotational speed sensor that is able to operate in an environment containing contamination while overcoming some or all of the above-mentioned shortcomings of the prior art.